1. Technical Field
The present disclosure relates to linear light-emitting diode (LED) lamps that operate with linear tube lamp fixtures configured to electrically connect to either an electronic ballast or AC mains, and more particularly to a universal, shock and fire hazard-free linear LED tube lamp with an arc and shock prevention mechanism.
2. Description of the Related Art
Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock and fire become especially important and need to be well addressed.
In today's retrofit application of a linear LED tube (LLT) lamp to replace an existing fluorescent tube, consumers may choose either to adopt a ballast-compatible LLT lamp with an existing ballast used to operate the fluorescent tube or to employ an AC mains-operable LED lamp by removing/bypassing the ballast. Either application has its advantages and disadvantages. In the former case, although the ballast consumes extra power, it is straightforward to replace the fluorescent tube without rewiring, which consumers may have a first impression that it is the best alternative to fluorescent tube lamps. But the fact is that total cost of ownership for this approach is high regardless of very low initial cost. For example, the ballast-compatible LLT lamps work only with particular types of ballasts. If the existing ballast is not compatible with the ballast-compatible LLT lamp, the consumers will have to replace the ballast. Some facilities built long time ago incorporate different types of fixtures, which requires extensive labor for both identifying ballasts and replacing incompatible ones. Moreover, a ballast-compatible LLT lamp can operate longer than the ballast. When an old ballast fails, a new ballast will be needed to replace in order to keep the ballast-compatible LLT lamps working. Maintenance will be complicated, sometimes for lamps and sometimes for ballasts. The incurred cost will preponderate over the initial cost savings by changeover to the ballast-compatible LLT lamps for hundreds of fixtures throughout a facility. When the ballast in a fixture dies, all the ballast-compatible tube lamps in the fixture go out until the ballast is replaced. In addition, replacing a failed ballast requires a certified electrician. The labor costs and long-term maintenance costs will be unacceptable to end users. From energy saving point of view, a ballast constantly draws power, even when the ballast-compatible LLT lamps are dead or not installed. In this sense, any energy saved while using the ballast-compatible LLT lamps becomes meaningless with the constant energy use by the ballast. In the long run, ballast-compatible LLT lamps are more expensive and less efficient than self-sustaining AC mains-operable LLT lamps.
On the contrary, an AC mains-operable LLT lamp does not require a ballast to operate. Before use of an AC mains-operable LLT lamp, the ballast in a fixture must be removed or bypassed. Removing or bypassing the ballast does not require an electrician and can be replaced by end users. Each AC mains-operable LLT lamp is self-sustaining. If one AC mains-operable tube lamp in a fixture goes out, other lamps in the fixture are not affected. Once installed, the AC mains-operable LLT lamps will only need to be replaced after 50,000 hours. In view of above advantages and disadvantages of both ballast-compatible LLT lamps and AC mains-operable LLT lamps, it seems that market needs a most cost-effective solution by using a universal LLT lamp that can be used with the AC mains and is compatible with an electronic ballast so that LLT lamp users can save an initial cost by changeover to such a universal LLT lamp followed by retrofitting the lamp fixture to be used with the AC mains when the ballast dies.
Electronic ballasts have several different types. However in the US, instant-start electronic ballasts are most popular in lamp fixtures because they are more efficient and less expensive than other types of electronic ballasts and have few wires for installation. Nevertheless, it is better for the ballast-compatible LLT lamp to be compatible with either instant-start or rapid-start electronic ballasts. In the context hereafter, the instant-start electronic ballast will be referred to when a ballast is mentioned unless a rapid-start electronic ballast is explicitly stated.
In the U.S. patent application Ser. No. 14/688,841, filed Apr. 16, 2015, two shock prevention switches and an all-in-one driving circuit are adopted in an LLT lamp such that AC power from either an electronic ballast or the AC mains can operate the lamp without operational uncertainty and electric shock hazards. In other words, no matter what a lamp fixture is configured as the AC mains or an electronic ballast compatible fashion, the LLT lamp automatically detects configurations and works for either one. All of such LLT lamps, no matter whether AC mains-operable or ballast compatible, are electrically wired as double-ended and have one construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LLT lamps always fails a safety test, which measures through lamp leakage current. Because an AC-mains voltage applies to both opposite ends of the tube when connected to a power source, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present a risk of shock during re-lamping. Due to this potential shock risk to the person who replaces the LLT lamps in an existing fluorescent tube fixture, Underwriters Laboratories (UL) uses its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do a current leakage test and to determine if the LLT lamps meet the consumer safety requirement. Although the LLT lamps used with an electronic ballast can pass the current leakage test, some kinds of electric shock hazards do exist. Experimental results show that the skin of the person who touches an exposed bi-pin may be burned due to such an electric shock. Fortunately, a mechanism of double shock prevention switches used in applications with the AC mains is also effective in applications with the ballasts to prevent the electric shock from happening, thus protecting consumers from such a hazard, no matter whether input voltage is from the AC mains or the electronic ballast. Therefore, a universal LLT lamp that can work with either the AC mains or the electronic ballast makes sense. The effectiveness of using double shock prevention switches for applications in the AC mains has been well addressed in U.S. Pat. No. 8,147,091, issued on Apr. 3, 2012. However, a conventional shock prevention switch has an inherent issue related to electric arc when operated with an electronic ballast. Unlike an AC voltage of 120 or 277 V/50-60 Hz from the AC mains, the output AC voltage and current from an electronic ballast presents a negative resistance characteristic. The feature that originally supports fluorescent tube to function properly becomes extremely detrimental to the conventional shock prevention switch due to electric arc likely occurring between two electrical contacts that have a high electrical potential difference with a high frequency, such as 600 V/50 kHz. Once a consumer fails to follow installation instructions to install or uninstall linear LED tube lamps such that one of two ends of the tube is in the fixture socket connected to a powered electronic ballast, and the other end is tweaked to connect to or disconnect from the associated socket, an internal arcing may occur between the electrical contacts in the associated switch. The arcing even in a short period such as several seconds can generate high heat, burning and melting electrical contacts and neighboring plastic enclosures, creating a fire hazard. The AC voltage of 120 or 277 V/50˜60 Hz from the AC mains does not have such an issue because its voltage is relatively low compared with a ballast output voltage of 600 V. Moreover, the AC frequency of 60 Hz automatically extinguishes an arc every 1/60 seconds, if existed. That is why a utility switch can be used in an electrical appliance to turn power on and off without any problem. However when used with an electronic ballast, the electrical contacts used in the conventional shock prevention switch can easily be burned out due to a high-voltage and high-frequency arcing introduced between each gap of each pair of the electrical contacts in the conventional shock prevention switch when someone tries to abusively tweak to remove a tube lamp from a fixture with a ballast that has a power on it. Although such a situation is rare, an internal arcing and burning, if occurred, does cause burning and even welding of the electrical contacts and melting the plastic enclosure, so called internal fire, creating consumer safety issues. A conventional approach to suppress an arc in a switch is to use a snubber circuit comprising a capacitor and a resistor or a metal oxide varistor across each pair of switch contacts to supply a path for transient current. The approach, however, can only lessen the situation to a limited degree. Besides, the use of such electronic components results in an increase of leakage current, counteracting the effect of electric shock prevention. Furthermore, the electrical contacts in the conventional shock prevention switch continuously deteriorate due to contact surface material ionization and even arcing. Depending on how frequent a tube lamp is to be inserted into or removed from a fixture, the switch contact life is continuously shortened, whereas the metal oxide varistor or components used in the snubber circuit absorb part of the energy in transients during switch on and off and are heated up. The high temperature degrades the electronic components, creating a long-term reliability problem. Eventually, when the conventional shock prevention switches lose their functions, they can no longer protect a person from being electrically shocked during relamping. Consumerism is especially important today. It is therefore the purpose of this invention to disclose an arc prevention mechanism in each of shock prevention switches used in an LED tube lamp operating with an electronic ballast.